Process and apparatus for separating a mixture of particles according to size



July 24, 1962 F. J FONTEIN 3,045,823

APPARATUS PROCESS AND FOR SEPARAT A MIXTURE OF PARTICLES ACCORDING TO ZE Filed Sept. 2, 1958 FIG.2 m 117 This invention relates to particle separation and more particularly to improvements in method and apparatus for separating particles suspended in a liquid.

In my copending application, Serial No. 475,251, filed December 14, 1954, which matured into Patent No. 2,916,142, there is disclosed a process which comprises the steps of: establishing at a feeding position a layer formation flow of particles and a suspending liquid and supporting one surface thereof; interrupting the support of the one surface of said layer formation flow for a fixed distance measured in the direction of flow; blocking the flow of an incremental layer, having a thickness substantially less than said fixed distance of support interruption, from the one surface of the unsupported layer formation flow so that the particles in the blocked incremental layer "of a size less than twice the thickness of the incremental layer together with a substantial portion of the suspending liquid in said incremental layer will be separated from the layer formation flow; supporting the one surface of the remainder of the layer formation flow a second fixed distance measured in the direction of flow; and successively repeating the steps of interrupting the support, blocking the flow of an incremental layer, and supporting the re mainder of said layer formation flow, between said feeding position and a discharge position throughout a path generally conforming to a surface generated by moving a line parallel to itself so that a given point on the line moves in a plane perpendicular to the moving line while maintaining a predetermined minimum velocity gradient in said layer formation flow between said feeding position and said discharge position.

According to the aforementioned specification, the suspension to be separated is fed tangentially to the concave side of a curved screening deck, in a direction perpendicular to the generatrices thereof. The screening deck consists of a curved plate having circular or slot-like apertures, the greatest dimension of the latter being parallel to the generatrices of the screening deck, or of bars substantially rectangular or trapezium-shaped in section, which bars are disposed in a direction parallel to the generatrices of the screening deck. I a

It is well known that by means of an apparatus equipped with a screening deck as described above, which apparatus is known by the name of sieve bend, a separation can be carried out in which substantially all of the solid particles taken along through the openings by the greater part of the liquid are not greater than half the width of the slot-like apertures or than one quarter of the diameter of the circular apertures, so that the danger of blockage is very small. The good operation of the sieve bend is to be ascribed to the phenomenon that, as a result of the high speed at which the suspension travels on the screening deck, each time the suspension crosses a slot-like aperture, a thin layer of liquidis separated from it, which layer is considerably thinner than the slot width and is in the main not thicker than A of the slot width; particles which are embedded for at least half of their diameter in this layer are entrained by it, with the consequence that the maximum diameter of a particle removed through a slot is not greater than half the slot width.

It has. been found that when, a classification is carried 3,045, 23 Patented July 24, 1962 in which the same pressure is created on both sides of the surface, the classification is in some places finer than in others, so that a certain non-uniformity of the classification over the surface is observed. Probably, this phenomenon is to be ascribed to liquid currents from the space adjoining the feed side of the surface to the space adjoining the other side, and vice versa. A liquid current from the feed side of the surface to the throughput side occurring at a certain spot will cause a coarser classification at this spot, whereas a current in the reverse direction will cause a finer classification. The feed side is'to be understood in this description as the surface of the classifying device on which the suspensionflows. is

It is an object of the subject invention not only to eliminate the above-mentioned disadvantages, but also to provide a control at will, of the fineness of the classification.

Another object of the present invention is the provision of a method of separating particles suspended in a liquid in which the ratio between the overpressures or underpressures prevailing on both sides of the separating surface in at least one part thereof can be controlled so that the fineness'of separation can be regulated within certain limits.

A further object of the present invention is the provision 'of an apparatus for separating particles suspended in a liquid which is provided with means fo'r'controlling the ratio between the overpressures or underpressures on both sides of the separating surface of the apparatus in at least one part thereof so that the fineness of separation can be regulated within certain limits.

Still another object of the present invention is the provision of an apparatus of the type described having improved means for controlling the pressures on opposite sides of the screening surfaces.

These and other objects of the present invention will become more apparent during .the course of the following detailed description and appended claims. 7

The invention may best be understood with reference to the accompanying drawings wherein illustrative embodiments are shown. i

, In the drawings:

FIGURE 1 is -a diagram of an embodiment of an apparatus according to the invention;

FIGURE 2 is a diagram of another embodiment;

. FIGURE 3 is a diagram of still another embodiment, with a curved screen deck. a

Referring now more particularly to the drawings, there is shown in FIGURE 1 a diagram of an apparatus embodying the principles of the present invention, which includes a screening deck 1, having a screening surface which faces downwardly. Surrounding the screening surface is a housing 2, the side walls of which are joined to the sides of the screening deck. Adjoining the screen.- ing deck and housing 2 atone end is a feed chamber 3, and a discharge chamber 4 for the overflow fraction is disposed at the opposite end thereof. The chambers 3 and 4- are preferably provided with shutoff valves 5 and 6, respectively. The housing 2. includes a substantially horizontal wall 7 which is disposed at a greater distance from the screening deck near the feed end thereof than near the delivery end. On the throughput side of the screen deck 1 there is a plurality of chambers '8 having walls,9-which are joined to the throughput side of the screening deck, the arrangement heingsuch that all the suspension flowing through the apertures in the screen deck is collected in these chambers. Each chamber is provided with a discharge pipe 10, through which the throughput canbe controlled, e.g., by means of a valve 11.

out under pressure over a sieve bend in a closed housing By controlling the amount of throughput through each valve 11, the extremely small pressure diiferences prevailing at both faces of the screen may be controlled.

The suspension to be screened is supplied at A. The oversize or overflow fraction is discharged at B, the undersize or throughput fractions are discharged at C.

Example I A suspension of sand in water was supplied under an overpressure of, for example, one atmosphere, to the feed side of a flat sieve bend having a slot width of 0.4 mm., a screen length of 760 mm., a screen width of 200 mm. and a distance between wall 7 and screening deck varying from 15 to 7 /2 mm. At the side of the chamber 8 there is maintained back pressure somewhat greater than the aforesaid pressure at the feed side so as to produce a thinner layer on the feed side with a smaller particle size. The feed was 34.4 cu. m./h., the throughput 9.6 cu. rn./h.

The screening results are given in the table below, in which the screening results measured in the eight successive chambers are stated separately.

pressure under the screen deck was kept the same, which meant a heightening of the pressure difference, it was found that the 50% particle size increased to 88 14, the 95% size of 142 The experiments proved that by a difierentation the pressure difference between the feed side and the throughput side of the screen deck the particle size of separation can be altered.

Referring now more particularly to FIGURE 2, there is shown a modified apparatus embodying the principles of the present invention which includes a screening deck 110, preferably constructed of bars perpendicular to the plane of the drawing or of a perforated plate, by preference one having slot-like openings normal to the plane of the drawing. The screening deck 110 divides a closed housing, having an upper substantially horizontal wall 111, into two compartments 112 and 1 13, the part 112 being on th e upper feed side and the part 113 on the lower throughput side of the screening deck. Adjoining the throughput side of the screening deck there are chambers Throughput in chambers Sample Overnew 1 2 3 4 5 6 7 8 Concentration in glliter 27 12 15 18 17 19 21 18 508 Grain size Percent By separately collecting the throughput fractions of the chambers and taking in account the measured concentrations of the said fraction, the average throughput of the screens can be determined. This average throughput multiplied with the total throughput provides the total amount of solid particles in the throughput. The total amount of solid particles in the overflow is found by multiplying the concentration of the overflow-fraction with the capacity of the overflow-fraction By making a graph, wherein the vertical axis indicates the percentage and the horizontal axis the size of the particles a curve is formed, wherein from every grain size the percentage present in the overflow fraction can be.

determined, such graphs are well known in the art of screening.

The 95% particle measured 127;, the particle /1,. At equal pressures at both faces of the screen, e.g. when the screen is positioned in the open atmosphere, if no back pressure had been exerted on the chambers 8, the particle would have measured about 200a. By 50% and 95% particles is to be understood the particle sizes of which 50% and 95%, respectively, get into the overflow fraction.

Example II In an experimental installation having a screen length of 75 mm. and a screen width of 2.35 mm, a slot width of 0.2 mm., a bar width of 0.9 mm, a suspension of coal in water was passed over the screen surface at a feed pressure of 0.2 atm. gauge, while equal overpressures were maintained at both sides of the screen deck.

The feed amounted to 35.48 cu. rn./h., the solids concentration in it being 69.4 g./l., so that the supply of solid substance amounted to 2.469 t./h.

The throughout fraction amounted to 0.16 cu. m./h., the solids concentration was 7.9 g./l., and the discharge of solid substance was 0.001 t./h. The size of the 50% particle was 76a, that of the 95 particle 1331.0. When the feed was increased to 37.17 cu. m./h., while the over- 114 adjoining each other and extending from the feed end to the delivery end, each chamber having a discharge aperture 115, e.g., a slot, whose surface is at most equal to the total of the surfaces of the slots in the corresponding part of the screening deck. The slots 115 all open into the space 113. Preferably, the area of the discharge apertures will be such that these apertures can discharge the amount of suspension under normal conditions separated off by the screen apertures, without a back pressure being created in the chambers 114. Thus, the generation of counter currents from the space 113 to the chambers 114 is prevented. As shown in the drawing, the space 112 has its widest passage area near the feed end of the screen deck and its narrowest passage area near the delivery end, the area decreasing gradually. The passage area of the space 113 increases correspondingly. As a result, the formation of eddies in the spaces 112 and 113 is largely avoided. The feed and the pressures in the spaces 112 and 113 are controlled by any suitable means, such as valves 116, 117, and 118.

The apparatus according to the invention prevents the formation of a current from the delivery end of the space 112 through the screening deck and the space 113 to the feed end thereof, and back through the screening deck to the feed end of the space 112. -Moreover, the apparatus provides the possibility of controlling the fineness of the screening through control of the pressure diflerence between the spaces 113 and 112.

It is not essential that the chambers 114 have the same shape as shown in the drawing. Several variations are possible, without departing from the invention.

Referring now more particularly to FIGURE 3, there is shown an apparatus similar to that shown in FIG- URE 2, except that the screening deck 1 10 is cylindrically bent, as indicated at 110 and the chambers are correspondingly shaped as indicated at 112' and 113'. The apparatus shown in FIGURE 3 also includes the remaining elements similar to those of the apparatus shown in FIGURE 2 designated by the corresponding primed numeral. Thus, the apparatus of FIGURE 3 includes a cylindrical wall 111', a plurality of chambers 114', slots an; M-

and.

115' and Valves 116', 117' and 118', all related in the same manner described above in regard to FIGURE 2.

It can thus be seen that the fineness of separation in one or more parts of the separating surface in all the embodiments shown is controlled by means of a control of the ratio between the overpressures or underpressures prevailing on both sides of the separating surface in this part or parts. In this way, the classification over the entire separating surface can be controlled, while the fineness can be regulated within certain limits in one and the same apparatus.

It can also be seen that a rise in the pressure prevailing on the throughput side causes a slight raising of the liquid layer flowing across the opening, so that the lowermost layer which is separated off becomes thinner and consequently smaller particles are entrained. When the pressure on thefeed side is raised with respect to the pressure on the throughput side the opposite will happen.

Preferably, the pressure ratio is controlled by collecting the throughput fractionsflowing through the abovementioned part or parts in separate spaces adjoining each of these parts, and controlling the discharge of the abovernentioned fraction or fractions from this space or spaces.

In general, the apparatus of the present invention utilizes a closed housing provided with feed apertures and controllable discharge apertures, which housing. is divided into two compartments by a classifying device constituted by a plate provided with circular or slot-like apertures or by parallel bars of rectangular or trapezium-shaped section, separated by slot-like openings, said apparatus further comprising means for supplying the material to be classified, which means deliver the material in the direction towards and parallel to the feed end of the classifying device and onto this feed end in a sense perpendicular to the slots, means for discharging the separated fractions, and means for creating an overpressure in the two parts of the housing, as described,

e.g., in the above said patent application. According to the invention thereare means, at least extending over a part of the classifying device and bounding on the throughput side thereof, for collecting and controlling the amount of material flowing through this part. These means for collecting and controlling may, according to the invention, consist of one or more collecting chamhers, the side walls of which form the bounding planes of the above-mentioned parts, each of which chambers is provided with at least one discharge aperture.

To insure good operation of the apparatus according to the invention, it is necessary that from every chamber an amount of suspension can flow that is of the same order of magnitude as the amount which would flow through the apertures of the part of the classifying device lying over the chamber, if the classifying device were installed in the open atmosphere while the suspension were passed across the classifying device at the same rate.

To this end, the chambers and/ or their discharge apertures must be given such a shape that through these discharge apertures the above-mentioned amount of material can flow. This may be attained by seeing to it that in the chambers a sufiiciently high resistance is created when the suspension is flowing through, or by giving the apertures in the chambers the required size.

In one embodiment of the invention the part of the housing adjoining the throughput side of the classifying device is divided by means of partitions into a number bounding one. chamber is kept as constant as possible, as pressure differences in the region over a chamber have to be kept small. Also, care should be taken that the flow profile on the feed side of the classifying device over a chamber is sufiiciently high as compared with the length, so that the pressure difference between the initial and final parts of the profile over a chamber cannot become unduly high. However, eddy currents in this part of the housing should also be guarded against. Favorable dimensions are such at which the area of the slots of a chamber is, at most, four times the mean passage area over this chamber at the feed side of the screening deck.

The invention can be applied not only to sieve bends which are enclosed in a housing, and in which, as is known, the suspension is fed tangentially onto the concave side of the screening deck in a direction perpendicular to the generatrices of the screening deck and in which the slot-like apertures are parallel to the generatrices, while the radius of curvature of the screening deck may be infinite. It may also be applied to classifying devices whose section perpendicular to the direction of flow of the material has a curved shape and in which, if the device has slot-like openings, the slots are perpendicular to the direction of flow of the material. Care will have to be taken that in a flow profile the height over the screening deck is about the same in all places.

With the apparatus according to the invention it is not necessary that the feed side of the separating deck faces upwards. For instance, if a fiat separating deck is used as shown in FIGURE 1, this may be so placed that the feed side faces down and the throughput side faces It thus will be seen that the objects of this invention have been fully and effectively accomplished. It will n-be realized, however, that the foregoing specific embodiof chambers each of which is provided with one or more ment has been shown and described only for the purpose .of illustrating the principles of this invention and is subject to extensive change without departure from such principles. Therefore, this invention includes all modifications encompassed within the spirit and scope of the following claims.

I claim:

1. A process of separating a mixture of particles according to size which comprises the steps of: establishing at a feeding position a layer formation flow of particles and a suspending liquid and supporting one surface thereof; interrupting the support of the one surface of said layer formation flow for a fixed distance measured in the direction of flow; blocking the flow of an incrementallayer having a thickness substantially less than said fixed distance of support interruption from the one surface of the unsupported layer formation flow so that the particles in the blocked incremental layer of a size less than twice the thickness of the incremental layer together with a substantial portion of the suspending liquid in said incremental layer will be separated from the layer formation flow; supporting the one surface of the remainder of the layer formation flow a second fixed distance measured in the direction of flow; successively repeating the steps of interrupting the support, blocking the flow of an incremental layer, and supporting the remainder of said layer formation flow between said feeding position and a discharge position throughout a smooth path generally conforming to a surface generated by moving a line parallel to itself so that a given point on the line moves in a plane perpendicular to the moving line while maintaining a predetermined minimum velocity gradient in said layer formation flow between said feeding posi tion and said discharge position confining the incremental layers and associated liquid separated from said layer formation flow at a plurality of positions throughout said path, maintaining pressures in the layer and in the separated incremental layers and associated liquid diifering from the atmospheric pressure and differing in the layer on the one side and the incremental layers and associated liquid at the other side and controlling in at least one position along said path said last differences by controlling at least one of the separated, emanating volumes of the said layer.

2. Apparatus for separating a mixture of particles according to size comprising a screening deck having a screening surface generally conforming to a smooth surface generated by moving a line perpendicular to a fiat plane along a line in said plane, a feed end at one end of said surface and a discharge end at the other end of said surface, so that material fed onto said surface travels along the screening line in a general direction perpendicular to said moving line, said surface being provided with apertures the dimensions whereof measured in the direction of flow of the material to be screened not exceeding the dimensions perpendicular to said direction, each aperture being defined by at least one surface facing in a direction opposed to the local direction of flow of the material and disposed in a plane transverse relative to the screening surface, a housing surrounding said screening deck on the screening side thereof and tapering toward the screening deck from the feed end to the discharge end thereof to provide a continually reduced cross-sectional area, means for feeding particles and a suspending liquid at a pressure different from the atmospheric pressure adjacent the feed end of said screening deck and past the screening surface in confined relation to said housing so that the surface defining said apertures facing in the direction of the feed end will engage incremental layers of a thickness substantially less than the width of the associated screening and particles in the incremental layers of a size less than twice the thickness thereof and a substantial portion of the suspending liquid will pass through the apertures and the remaining particles and liquid will pass on to the discharge end of the screening deck, adjustable means for discharging from said housing the particles and liquid remaining on the screening surface at the discharge end thereof, and means surrounding at least a portion of said screening deck on the side thereof opposite from said screening surface for collecting and controlling the amount and size of particles and suspending liquid passing through the associated screening apertures.

33. Apparatus as defined in claim 2 wherein said last mentioned means comprises a collecting chamber having sides defining the boundary of said screening deck portion, said chamber having at least one discharge opening therein.

4. Apparatus for separating a mixture of particles according to size comprising a screening deck having a screening surface generally conforming to a smooth surface generated by moving a line perpendicular to a flat plane along a line in said plane, a feed end at one end of said surface and a discharge end at the other end of said surface, so that material fed onto said surface travels along the screening line in a general direction perpendicular to said moving line, said surface being provided with apertures the dimensions whereof measured in the direction of flow of thematerial to be screened not exceed ing the dimensions perpendicular to said direction, each aperture being defined by at least one surface facing in a direction opposed to the local direction of flow of the material and disposed in a plane transverse relative to the screening surface, a housing surrounding said screening deck on the screening side thereof, means for feeding particles and a suspending liquid at a pressure different from the atmospheric pressure adjacent the feed end of said screening deck and past the screening surface in confined relation to said housing so that the surface defining said apertures facing in the direction of the feed end will engage incremental layers of a thickness substantially less than the width of the associated screening and particles in the incremental layers of a size less than twice the thickness thereof and a substantial portion of the suspending liquid will pass through the apertures and the remaining particles and liquid will pass on to the discharge end of the screening deck, adjustable means for discharging from said housing the particles and liquid remaining on the screening surface at the discharge end thereof, and means surrounding at least a portion of said screening deck on the side thereof opposite from said screening surface for collecting and controlling the amount and size of particles and suspending liquid passing through the associated screening apertures and including a plurality of collecting chambers between the feed end and discharge end of said screening deck, each of said chambers having a discharge opening.

5. Apparatus as defined in claim 4 including means for controlling the area of each discharge opening.

6. Apparatus as defined in claim 4 wherein each discharge opening is so shaped that the amount of particles and suspending liquid that can flow therethrough is of the same order of magnitude as the amount which would flow through the associated screening apertures when operating at atmospheric pressure.

7. Apparatus as defined in claim 4 wherein the area of the screening apertures associated with each chamber is at most equal to four times the mean area under the chamber at the feed side of the screening deck.

8. Apparatus as defined in claim 4 wherein said housing includes a portion surrounding the discharge openings of said chambers, said housing portion having a passage area which gradually increases from the feed end of said screening deck to the delivery end thereof, the area at any position being such that it is at least equal to the joint areas of the chamber discharge openings lying upstream from such position.

References Cited in the file of this patent UNITED STATES PATENTS 1,135,304 Liggett et al Apr. 13, 1915 2,912,109 Mitchell Nov. 10, 1959 2,916,142 Fontein Dec. 8, 1959 FOREIGN PATENTS 722,195 Great Britain Jan. 19, 1955 

